As physical properties of high-efficiency, environment-friendly and high-performance tires are required for reducing discharge of carbon dioxide and improving fuel economy, and the like, tire materials that meet such needs are actively being developed. Particularly, styrene-butadiene rubber obtained by solution polymerization (hereinafter, referred to as SSBR), unlike emulsion polymerization, is easy to change structure, and reduces movement of chain ends by bonding or modifying the chain ends and increases bonding force with carbon black, so that it has been used as a rubber material for tire treads. In addition, as silica filling materials are developed, low rolling resistance and high road surface braking force can be obtained at the same time, but to this end, a technique of combining hydrophilic silica with the SSBR having hydrophobicity and dispensing them therein is needed.
Such a method includes a method of surrounding silica particles themselves with a hydrophobic substance, a method of using a coupling agent between the silica and the SSBR, and the like. Recently, techniques for introducing a moiety capable of reacting and bonding with silica or a moiety for serving to assist this into the SSBR polymer chains themselves by using a modification initiator, a modifying monomer or a modifying agent, and the like upon SSBR anionic polymerization have been developed. In particular, the modification initiator is used as an essential material for preparing such a modified SSBR by initiating anionic polymerization and serving to introduce a functional group into one end of the chain.
Among anionic polymerization initiators used upon synthesizing such an SSBR, hexamethylene lithium (HMI-Li) initiator is prepared by reaction of hexamethyleneimine (HMI) and n-butyllithium (BuLi, NBL) as in the following reaction scheme.

However, HMI-Li has a low solubility in a solvent to fall into precipitation over time and also has a problem that the reactivity is lower than that of BuLi, although it can be used as an initiator. In order to solve this disadvantage, conventionally, the polymerization initiator was prepared by further reacting a conjugated diene (R) such as isoprene (IP) or 1,3-butadiene (BD) after being subjected to Reaction Scheme 1, as in the following Reaction Scheme 2. The initiator may increase the solubility in an organic solvent by further attaching such conjugated dienes to perform a stable reaction, and also has the reactivity higher than that of HMI-Li as an initiator, which is sufficient to initiate anionic polymerization.

In Reaction Scheme 2, n is an integer from 1 to 100.
However, the modification initiator thus prepared is unstable over time and falls into precipitation, or is inactivated by bonding a very small amount of oxygen with water. Therefore, the existing process of preparing the above polymerization initiator in a batch-wise manner and then introducing it into the polymerization reaction necessarily requires a storage step of the modification initiator, thereby bringing about the aforementioned disadvantages. This may adversely affect the post-processes to be a factor for lowering physical properties of the finally synthesized SSBR, which makes it difficult to maintain a constant quality.
In the prior art, an anionic polymerization initiator was prepared by a batch-wise process and then used to prepare a solution-polymerized SSBR. Or the preparation of an anionic polymerization initiator and a solution-polymerized styrene-butadiene rubber in a batch-wise reactor was carried out simultaneously in one pot.
In the case of the former, the storage step of the modification initiator is necessarily required, and during the period of storing the already synthesized initiator its anions are reacted with various scavengers such as moisture and air and thus the initiator loses its activity. This may adversely affect the post-processes to be a factor for lowering the physical properties of the finally synthesized SSBR, which makes it difficult to maintain a constant quality. In the case of the latter, it is a process of performing the polymerization reaction in the same batch-wise reactor simultaneously with the initiator synthesis reaction, whereby the problem of storage could be solved. However, it is difficult to confirm whether the modification initiator is properly synthesized, and the physical properties are also deteriorated more than the case of adding the synthesized initiator. In addition, in all the conventional batch-wise processes, by-products are produced while raw materials are directly introduced, mixed and reacted, or reverse reactions occur to generate unreacted products, and as a result, there is also a problem that a polymerization yield is lowered.